Fuel cells are classified into several types according to types of electrolytes or electrodes. Typical types are alkaline types, phosphoric acid types, molten carbonate types, solid electrolyte types and polymer electrolyte types. Of these, polymer electrolyte fuel cells that can operate at temperatures ranging from low temperatures (about −40° C.) to about 120° C. have been attracting attention and have been recently developed and practically used as power sources for low pollution automobiles.
The polymer electrolyte fuel cells are expected to be used as automobile drive sources or stationary power sources. The use in these applications requires long-term durability.
The polymer electrolyte fuel cell has a structure in which a solid polymer electrolyte is sandwiched between an anode and a cathode, and a fuel is fed to the anode, and oxygen or air is supplied to the cathode, whereby oxygen is reduced at the cathode to produce electricity. The fuel is primarily hydrogen, methanol or the like.
Conventionally, to increase the reaction rate in fuel cells and enhance the energy conversion efficiency of fuel cells, a layer containing a catalyst (hereinafter, also referred to as a “fuel cell catalyst layer”) is formed on the surface of a cathode (an air electrode) or an anode (a fuel electrode) of fuel cells.
As this catalyst, noble metals are generally used. Of noble metals, platinum has been primarily used, which is stable at high potential and has high activity. However, since platinum is expensive and exists in a limited amount, the development of alternative fuel cell catalysts has been desired.
As a catalyst alternative to platinum, materials containing nonmetals such as carbon, nitrogen and boron have been recently attracting attention. The materials containing these nonmetals are more inexpensive and are abundant compared with noble metals such as platinum.
Non-Patent Document 1 reports that zirconium-based ZrOxN compounds show oxygen reducing activity.
Patent Document 1 discloses, as platinum-alternative materials, oxygen-reducing electrode materials containing a nitride of one or more elements selected from Groups 4, 5 and 14 in the long periodic table.
However, the materials containing these nonmetals do not provide sufficient oxygen reducing activity for practical use as catalysts.
Patent Document 2 studies the possibility of using oxides having a perovskite structure containing two or more kinds of metals as platinum-alternative catalysts. However, as shown in its examples, the performance of the oxide does not exceed the performance of a carrier supplementing platinum, and thus sufficient activity is not achieved.